1. Field of the Invention
The present invention relates to an image forming apparatus equipped with a developing apparatus for use in an electrophotographic copying machine, a printer, a facsimile apparatus, etc., and in particular, to an image forming apparatus equipped with a developing apparatus using a developer containing toner and carrier.
2. Related Background Art
Conventionally, in an image forming apparatus, such as an electrophotographic copying machine, a developing apparatus effecting development by a two-component developing system using a two-component developer containing a mixture of toner and carrier, is generally widely used. In particular, in the case of color image forming apparatuses, from the viewpoint of image color tint, etc., many developing apparatuses use a two-component developer.
In a developing apparatus using a two-component developer, the toner is consumed by developing operation, and new toner is supplied in an amount corresponding to the consumed amount. In contrast, the carrier is not consumed but remains in a developing container, which is a developer accommodating portion of the developing apparatus. Thus, the carrier, which is agitated together with the toner, is degenerated by being agitated. The degeneration of the carrier occurs as a result of adhesion of toner to the surface of the carrier due to the friction when agitating the developer and to the compression during passage through the gap between a developing roller serving as a developer carrier for carrying developer to the developing position by rotating and a developer regulating member (developer regulating blade) for regulating the thickness of the toner layer on the developing roller. Degeneration of the carrier results in a deterioration in the charging performance of the developer. In view of this, it is general practice to periodically replace the carrier.
To replace the carrier, the developer is discharged through a developer discharging port provided in the developing apparatus, and the developing apparatus is filled with new developer. That is, the replacement of the carrier substantially means the total replacement of the developer in the developing apparatus, and in this process, the toner is naturally also renewed. During the replacement operation, it may occur that the toner, which is in the powder form, is scattered to stain the image forming apparatus and the periphery thereof.
In view of this, there has been proposed a developing apparatus in which the degeneration of the developer is restrained, so there is no need to perform the developer replacement operation (see, for example, Japanese Patent Publication No. H02-21519. The apparatus adopts a system generally referred to as a trickle system, in which a discharge port is provided at a predetermined height on a side wall of a housing constituting the developing container of the developing apparatus, and in which a developer containing toner and carrier is supplied to the developing apparatus, whereby surplus developer in the developing apparatus is successively discharged through the discharge port, thereby keeping the characteristics of the developer fixed (see, for example, Japanese Patent Application Laid-Open No. 2003-215903).
The above-mentioned conventional apparatus, however, has a problem in that, when the toner density of the developer to be replenished (replenishment agent) and the T/D ratio, which is the ratio of toner to developer, fluctuate, the toner replenishment accuracy deteriorates, with the result that the toner supply to the developing apparatus becomes deficient or excessive.
When the toner supply becomes deficient or excessive, the toner density of the developer in the developing apparatus fluctuates, which may lead to a fluctuation in the toner image density or scattering of toner from the developing apparatus.
Alternatively, due to a fluctuation in environmental conditions such as humidity, the electric charge of the toner (triboelectricity) fluctuates, which leads to a change in bulk density and a deterioration in the accuracy with which developer is supplied to the developing apparatus, with the result that the toner supply to the developing apparatus becomes deficient or excessive, thus involving the same problem as mentioned above.